1. Field of the Invention
The present invention relates to an antireflection film for effectively preventing external reflection and a display apparatus having the antireflection film and, more particularly, to an antireflection film having a two-layer structure.
2. Description of the Related Art
A glass plate is generally used as a substrate for a window glass, a show window, or the display surface of a display apparatus. This glass plate sometimes causes specular reflection of ambient light such as light from the sun and light from lighting units. For this reason, reflection often occurs, resulting in a deterioration in transparency. Especially in a display apparatus, if specular reflection occurs on the display surface, an image to be displayed on the display surface overlaps an a light source, a scene, and the like located near the apparatus and causing reflection on the display surface so as to cause a considerable deterioration in image quality.
In a conventional method of preventing such reflection, a single-layer or multilayer optical film, i.e., an antireflection film, is formed on the substrate surface to prevent external reflection by using interference of light.
As this antireflection film, a film called a 1/4-wave film is known. This 1/4-wave film will be described below.
When external reflection is to be prevented by a single-layer antireflection film, the following non-reflection conditions must be satisfied, provided that the refractive index of air is represented by n.sub.0 ; the refractive index of the thin film, n.sub.1 ; the refractive index of the substrate, n.sub.2 ; the thickness of the thin film, d; and the wavelength of light which is to be prevented from reflecting, .lambda.. EQU n.sub.1 d=.lambda./4 EQU n.sub.1.sup.2 =n.sub.0 n.sub.2 ( 1)
In equations (1), since the thickness of the thin film corresponds to 1/4 the wavelength of the light which is to be prevented from reflecting, the film is called a 1/4-wave film.
If equations (1) are satisfied, reflective index of the light having the wavelength .lambda. can be reduced to zero. If the substrate is composed of glass, n.sub.2 is 1.52 and the refractive index n.sub.0 of air is 1.00. Therefore, the refractive index n.sub.1 of the thin film must be set to be 1.23. The most practicable low-refractive-index material of currently known thin film materials is MgF.sub.2. The refractive index of MgF.sub.2 is 1.38, which is larger than the refractive index (n.sub.1 =1.23) defined by the non-reflection conditions. For this reason, it is impossible to completely prevent external reflection by using only a single layer consisting of MgF.sub.2.
Under the circumstances, attempts have been made to prevent reflection by forming a two-layer antireflection film consisting of lower and upper layers on a substrate. With this film, the following non-reflection conditions must be satisfied, provided that the refractive index of air is represented by n.sub.0 ; the refractive index of the upper layer, n.sub.3 ; the refractive index of the lower layer, n.sub.4 ; the refractive index of the substrate, n.sub.2 ; the thickness of the upper layer, d.sub.1 ; the thickness of the lower layer, d.sub.2 ; and the wavelength of light which is to be prevented from reflecting, .lambda.. EQU n.sub.3 d.sub.1 =.lambda./4 EQU n.sub.4 d.sub.2 =.lambda./4 EQU n.sub.2 n.sub.3.sup.2 =n.sub.0 n.sub.4.sup.2 ( 2)
According to equations (2), if the substrate is a glass plate, since n.sub.2 =1.52 and n.sub.0 =1.00, reflection can be prevented by selecting materials for the lower and upper layers such that a refractive index ratio n.sub.4 /n.sub.3 is set to be 1.23.
Such a two-layer antireflection film is disclosed in, e.g., Published Unexamined Japanese Patent Application No. 61-10043. This antireflection film has a high-refractive-index film consisting of a co-condensate of an alkoxide of Ti, Zr, or Si as a lower layer, and a low-refractive-index film consisting of a condensate of alkoxysilane or chlorosilane containing a polyfluoroalkyl group as an upper layer.
Such a two-layer antireflection film can prevent reflection of light rays at specific wavelengths which satisfy the non-reflection conditions but cannot substantially prevent reflection of light rays having wavelengths other than the specific wavelengths. FIG. 1 shows the spectral reflectances of a two-layer antireflection film which is designed such that a combination of the refractive indices of the lower and upper layers is set to satisfy the non-reflection conditions for a glass substrate having a refractive index of 1.52 so as to prevent reflection of a light ray having a wavelength .lambda..sub.0. Referring to FIG. 1, a curve 1 is obtained when the refractive index n.sub.3 (of the upper layer)=1.55 and the refractive index n.sub.4 (of the lower layer)=1.91, a curve 2 is obtained when the refractive index n.sub.3 =1.45 and the refractive index n.sub.4 =1.78 and a curve 3 is obtained when the refractive index n.sub.3 =1.38 and the refractive index n.sub.4 =1.70. As shown in FIG. 1, each reflectance characteristic curve is relatively steep. That is, the antireflection region in which the reflectance of the substrate having the antireflection film is smaller than that of the substrate itself is relatively narrow. In addition, the antireflection region is widened as lower refractive indices of the upper and lower layers are combined. As described above, the most practicable low-refractive-index material of currently known antireflection film materials is MgF.sub.2. With a conventional two-layer film, only an antireflection film having a reflectance characteristic represented by the curve 3 can be expected at best.
As described above, with the conventional two-layer antireflection film, an antireflection effect can only be obtained within a relatively narrow wavelength region. Furthermore, in a region other than a specific wavelength region within which reflection can be prevented, the reflectance may become higher than that of the substrate itself, resulting in glare of the screen. This tendency is conspicuous especially on the short-wavelength side, and bluish glare is caused on the display surface of a display apparatus.
It is known that reflection can be prevented in a wide region by using an antireflection film consisting of three or more layers instead of a two-layer film. More specifically, since the thickness of an antireflection film is determined by the wavelength of light, a multilayer antireflection film consisting of N layers can reduce the reflectances for light rays of N wavelengths. However, an increase in number of layers of an antireflection film leads to an increase in number of processes, a decrease in yield, an increase in cost, and the like, thus posing difficulties in terms of industrial applications.